GB2048498A

GB2048498A - A moving-coil indicating instrument for measuring an electric current

Info

Publication number: GB2048498A
Application number: GB8009138A
Authority: GB
Original assignee: Cgs Istrumenti Misura SpA
Current assignee: Cgs Istrumenti Misura SpA
Priority date: 1979-03-23
Filing date: 1980-03-18
Publication date: 1980-12-10
Also published as: DE3009878A1; FR2452112B3; BR8001694A; FR2452112A1; IT1111579B; IT7921249A0

Abstract

A moving-coil voltmeter has a ferromagnetic element 30 near the permanent magnet 6 of pole-pieces 2, 5. The permeability of element 30 has a high negative temperature coefficient, so that the amount of flux which is shunted by the element 30 decreases with increase in temperature. The flux available to link the moving coil 14 thus increases, which is intended to offset the decrease in coil current caused by increase in coil resistance. <IMAGE>

Description

SPECIFICATION A moving-coil indicating instrument, more particulary for measuring an electric current The invention relates to a moving-coil indicating instrument, more particularly for measuring an electric voltage, of the kind comprising two polepieces polarized by a permanent magnet and a movable assembly comprising a coil connected to one portion of one of the pole-pieces so as to be flowed through by part of the magnetic flux generated by the permanent magnet.

Known indicating instruments in use have a disadvantage in that the reading of the measured voltage varies with temperature.

If a given voltage is to be measured, the current flowing through the instrument varies with temperature owing to the variation in the resistance of the coil. Since the angular position of the movable assembly depends directly on the current flowing through the coil, it follows that if the temperature varies, the angular position varies, producing a consequent variation in the reading given by the instrument.

The invention is based on the problem of devising an indicating instrument having improved structural and functional characteristics so as to obviate the aforementioned disadvantage of the prior art.

This problem is solved by an instrument of the specified kind characterised in that is comprises an element made of ferromagnetic material extending near the permanent magnet so as to short-circuit part of the magnetic flux generated by the permanent magnet, the permeability of the ferromagnetic material forming the element having a temperature coefficient having a predetermined high value.

Other features and advantages of the indicating instrument according to the invention will be clear from the following description of a preferred embodiment, given by way of non-limitative example with reference to the single accompanying drawing, which is an exploded view of the instrument according to the invention.

The single accompanying drawing shows an indicating instrument 1 according to the invention comprising a first pole-piece 2 which is flat and made up of a toroidal portion 3 having an axis X-X and a heel-shaped portion 4.

Instrument 1 also comprises a second, plateshaped pole-piece 5 extending parallel to the first pole-piece 2.

A permanent plate-shaped magnet 6 is inserted between the pole-pieces in order to polarize them.

A plate 7 extends parallel to the second pole-piece 5 so as to hold the first pole-piece 2 in a central position between them.

Plate 7 and pole-piece 5 are interconnected by two columns 8,9 of equal height. More particularly, one end of columns 8 and 9 is keyed and force-fitted on to place 7. The second pole-piece 5 is secured to the other ends of the columns by screws 10 and 11.

Instrument 1 also comprises a movable assembly (general reference 12) rotatable through an angle around axis X-X along arrow M, comprising a substantially C-shaped support 13 and a coil 14 connected to the toroidal portion 3 of the first pole-piece 2 and intially mounted thereon along a slot 4a formed in the heel portion 4.

The movable assembly is rotatably held at ends 15a,15b of a pin 15 securely mounted on pole-piece 2.

One end of the C-shaped support 13 has conventional balancing means (general reference 17). At the other end of the C there are electric terminals 18, 19 for connecting to the internal ends of spiral springs 20, 21 made of conductive material.

The outer ends of springs 20, 21 are connected to electric terminals 22, 23 for supplying an electric current to be measured.

Terminals 22, 23 are held by a ring 24 of insulating material secured to plate 7 by sheets 25, 26.

Assembly 12 has a seat 27 adapted to receive a pointer (conventional and therefore not shown).

Instrument 1 also comprises a bridge place 28 secured at its ends to plate 7 at columns 8 and 9.

At its centre, plate 28 has a screw 29 made of ferromagnetic material and adjustable as necessary, for adjusting the bottom of the scale of the instrument.

The instrument 1 according to the invention comprises an element 30 of ferromagnetic material extending near the permanent magnet 6.

Element 30 is held against pole-piece 4 of bridge place 28 by a stirrup 31 made of plastics, which in turn is held in position by screw 29, which engages a seat 31a formed in stirrup 30.

The permeability of the ferromagnetic material forming the aforementioned element has a temperature coefficient having a predetermined high value.

More particularly the ferromagnetic material forming element 30 shows a high percentage decrease in permeability after a unit increase in temperature.

One example of such materials is thermoflux.

The operation of the instrument according to the invention will now be described. Terminals 23, 24 of instrument 1 are connected to a power system, so that the measuring voltage flows across the aforementioned terminals. The movable assembly is subjected to a torque. As is known, the torque is proportional to the current flowing through the coil and the magnetic flux through the coil.

When the temperature increases, the current decreases owing to the increase in resistance of the coil. There is a simultaneous increase in the flex through the coil. The reason is that the decrease in permeability of element 30 is high, so that the flux short-circuited by element 30 decreases considerably, so that an approximately equal increase in flux is made available for the coil. The result is that the torque on the movable assembly does not vary with temperature.

Consequently, the reading of the instrument is constant, in spite of temperature variations.

1. A moving-coil indicating instrument of the kind comprising two pole-pieces polarized by a permanent magnet and a movable assembly com

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION A moving-coil indicating instrument, more particulary for measuring an electric current The invention relates to a moving-coil indicating instrument, more particularly for measuring an electric voltage, of the kind comprising two polepieces polarized by a permanent magnet and a movable assembly comprising a coil connected to one portion of one of the pole-pieces so as to be flowed through by part of the magnetic flux generated by the permanent magnet. Known indicating instruments in use have a disadvantage in that the reading of the measured voltage varies with temperature. If a given voltage is to be measured, the current flowing through the instrument varies with temperature owing to the variation in the resistance of the coil. Since the angular position of the movable assembly depends directly on the current flowing through the coil, it follows that if the temperature varies, the angular position varies, producing a consequent variation in the reading given by the instrument. The invention is based on the problem of devising an indicating instrument having improved structural and functional characteristics so as to obviate the aforementioned disadvantage of the prior art. This problem is solved by an instrument of the specified kind characterised in that is comprises an element made of ferromagnetic material extending near the permanent magnet so as to short-circuit part of the magnetic flux generated by the permanent magnet, the permeability of the ferromagnetic material forming the element having a temperature coefficient having a predetermined high value. Other features and advantages of the indicating instrument according to the invention will be clear from the following description of a preferred embodiment, given by way of non-limitative example with reference to the single accompanying drawing, which is an exploded view of the instrument according to the invention. The single accompanying drawing shows an indicating instrument 1 according to the invention comprising a first pole-piece 2 which is flat and made up of a toroidal portion 3 having an axis X-X and a heel-shaped portion 4. Instrument 1 also comprises a second, plateshaped pole-piece 5 extending parallel to the first pole-piece 2. A permanent plate-shaped magnet 6 is inserted between the pole-pieces in order to polarize them. A plate 7 extends parallel to the second pole-piece 5 so as to hold the first pole-piece 2 in a central position between them. Plate 7 and pole-piece 5 are interconnected by two columns 8,9 of equal height. More particularly, one end of columns 8 and 9 is keyed and force-fitted on to place 7. The second pole-piece 5 is secured to the other ends of the columns by screws 10 and 11. Instrument 1 also comprises a movable assembly (general reference 12) rotatable through an angle around axis X-X along arrow M, comprising a substantially C-shaped support 13 and a coil 14 connected to the toroidal portion 3 of the first pole-piece 2 and intially mounted thereon along a slot 4a formed in the heel portion 4. The movable assembly is rotatably held at ends 15a,15b of a pin 15 securely mounted on pole-piece 2. One end of the C-shaped support 13 has conventional balancing means (general reference 17). At the other end of the C there are electric terminals 18, 19 for connecting to the internal ends of spiral springs 20, 21 made of conductive material. The outer ends of springs 20, 21 are connected to electric terminals 22, 23 for supplying an electric current to be measured. Terminals 22, 23 are held by a ring 24 of insulating material secured to plate 7 by sheets 25, 26. Assembly 12 has a seat 27 adapted to receive a pointer (conventional and therefore not shown). Instrument 1 also comprises a bridge place 28 secured at its ends to plate 7 at columns 8 and 9. At its centre, plate 28 has a screw 29 made of ferromagnetic material and adjustable as necessary, for adjusting the bottom of the scale of the instrument. The instrument 1 according to the invention comprises an element 30 of ferromagnetic material extending near the permanent magnet 6. Element 30 is held against pole-piece 4 of bridge place 28 by a stirrup 31 made of plastics, which in turn is held in position by screw 29, which engages a seat 31a formed in stirrup 30. The permeability of the ferromagnetic material forming the aforementioned element has a temperature coefficient having a predetermined high value. More particularly the ferromagnetic material forming element 30 shows a high percentage decrease in permeability after a unit increase in temperature. One example of such materials is thermoflux. The operation of the instrument according to the invention will now be described. Terminals 23, 24 of instrument 1 are connected to a power system, so that the measuring voltage flows across the aforementioned terminals. The movable assembly is subjected to a torque. As is known, the torque is proportional to the current flowing through the coil and the magnetic flux through the coil. When the temperature increases, the current decreases owing to the increase in resistance of the coil. There is a simultaneous increase in the flex through the coil. The reason is that the decrease in permeability of element 30 is high, so that the flux short-circuited by element 30 decreases considerably, so that an approximately equal increase in flux is made available for the coil. The result is that the torque on the movable assembly does not vary with temperature. Consequently, the reading of the instrument is constant, in spite of temperature variations. CLAIMS

1. A moving-coil indicating instrument of the kind comprising two pole-pieces polarized by a permanent magnet and a movable assembly com prising a coil connected to one portion of one of the pole-pieces so as to divide off part of the magnetic flux generated by the permanent magnet, characterised in that it comprises an element made of ferromagnetic material extending near the permanent magnet so as to short-circuit part of the magnetic flux generated by the permanent magnet, the permeability of the ferromagnetic material forming the element having a temperature coefficient having a predetermined high value.

2. An instrument according to Claim 1, characterised in that the material forming the element is thermoflux.

3. An instrument substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.